Negative electrode active material, preparation method thereof, and negative electrode and secondary battery including same

ABSTRACT

A negative electrode active material including natural graphite. The negative electrode active material has a ratio of D 90  to D 10 , which is D 90 /D 10 , of 2.0 to 2.2, a tap density of 1.11 g/cm 3  to 1.19 g/cm 3 , and a BET specific surface area of 2.02 m 2 /g to 2.30 m 2 /g.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Korean Patent Application No. 10-2019-0161897, filed on Dec. 6, 2019, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present invention relates to a negative electrode active material, and a method for preparing the same, and a negative electrode and a secondary battery including the same.

BACKGROUND ART

As the price of energy sources has increased due to the depletion of fossil fuels, and the interest in environmental pollution is amplified, eco-friendly alternative energy sources have become an indispensable factor for future life.

Particularly, as the technology development and demand for mobile devices have increased, the demand for secondary batteries as an eco-friendly energy source has been rapidly increased.

Typically, the secondary battery has used a lithium metal as a negative electrode of a secondary battery. However, since a risk of battery short circuit due to the formation of dendrite and battery explosion caused thereby became problematic, the use a carbon-based active material capable of intercalation and de-intercalation of reversible lithium ions and maintaining structural and electrical properties has emerged.

As the carbon-based active material, various types of carbon-based materials, such as artificial graphite, natural graphite, and hard carbon, have been applied. Among these, a graphite-based active material capable of ensuring the lifespan properties of a lithium secondary battery with excellent reversibility has been most widely used. Since the graphite-based active material has a low discharge voltage of −0.2 V compared to lithium, a battery using the graphite-based active material may exhibit a high discharge voltage of 3.6 V, thereby providing a number of advantages in terms of the energy density of a lithium battery.

Among the graphite-based active materials, natural graphite in particular has a higher output and capacity than other carbon-based active materials such as artificial graphite, and has excellent adhesion, and thus, has the advantage of reducing the amount of use of a binder, and the like, and implementing a high-capacity, high-density negative electrode. However, general natural graphite has too many internal pores compared to artificial graphite, and thus, has a problem in that the cycle swelling caused by an electrolyte side reaction becomes severe as charging and discharging continues, so that the use thereof may be limited despite the above-mentioned advantages.

In order to prevent the cycle expansion problem of natural graphite, a mixture in which natural graphite and artificial graphite are mixed in a certain ratio may be used as a negative electrode active material, but even in this case, the natural graphite is still poor in preventing cycle expansion caused by charging and discharging compared to artificial graphite. Therefore, it is not possible to sufficiently prevent the thickness expansion when the natural graphite is used in a negative electrode.

Thus, it is necessary to develop a negative electrode active material capable of preventing a cycle expansion problem while promoting the high output and capacity of natural graphite.

Although Japanese Patent Laid-Open Publication No. 4403327 discloses with respect to graphite powder for a negative electrode of a lithium-ion secondary battery, the disclosure does not provide an alternative to solve the aforementioned problems.

PRIOR ART DOCUMENT Patent Document

Japanese Patent Laid-Open Publication No. 4403327

DISCLOSURE OF THE INVENTION Technical Problem

An aspect of the present invention provides a negative electrode active material including natural graphite, which is capable of exhibiting excellent output and capacity properties of natural graphite while effectively preventing a cycle expansion problem caused by an electrolyte side reaction.

Another aspect of the present invention provides a method for preparing a negative electrode active material, wherein the method includes a process of pressurizing natural graphite by a Cold Isostatic Press method, thereby effectively preventing a cycle expansion problem of natural graphite.

Yet another aspect of the present invention provides a negative electrode and a secondary battery including the above-described negative electrode active material.

Technical Solution

According to an aspect of the present invention, there is provided a negative electrode active material including natural graphite, wherein the negative electrode active material has a ratio of D₉₀ to D₁₀, which is D₉₀/D₁₀, of 2.0 to 2.2; a tap density of 1.11 g/cm³ to 1.19 g/cm³; and a BET specific surface area of 2.02 m²/g to 2.30 m²/g.

According to another aspect of the present invention, there is provided a method for preparing a negative electrode active material, the method including pressurizing natural graphite by a cold isostatic press method and adjusting a D₉₀/D₁₀ of the pressurized natural graphite, which is a ratio of D₉₀ to D₁₀, to be 2.0 to 2.2, wherein the natural graphite having the adjusted D₉₀/D₁₀ has a BET specific surface area of 2.02 m²/g to 2.30 m²/g and a tap density of 1.11 g/cm³ to 1.19 g/cm³.

According to another aspect of the present invention, there is provided a negative electrode including a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer comprises the above-described negative electrode active material.

According to another aspect of the present invention, there is provided a secondary battery including the negative electrode described above, a positive electrode facing the negative electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte.

Advantageous Effects

A negative electrode active material of the present invention has the ratio of D10 and D90 in a particle diameter distribution, tap density, and BET specific surface area adjusted to a desired level in a specific range, and thus, when applied to a negative electrode, packing among active material particles may be smoothly achieved, and the problem of cycle expansion caused by an electrolyte side reaction and the problem of thickness expansion of a negative electrode active material layer may be effectively prevented. Accordingly, the output and capacity properties of natural graphite may be effectively exhibited, while improving the lifespan properties.

In addition, a method for preparing a negative electrode active material of the present invention pressurizes natural graphite and adjusts the particle size, tap density, and specific surface area of the active material by applying Cold Isostatic Press (CIP) for the implementation of the above-described negative electrode active material. Therefore, according to the preparation method of the present invention, micro-pores present inside natural graphite or a negative electrode active material may be reduced or removed to reduce the specific surface area of the active material, and packing between particles may be facilitated, so that the expansion problem of the active material according to the charging and discharging may be effectively prevented.

MODE FOR CARRYING OUT THE INVENTION

It will be understood that words or terms used in the specification and claims of the present invention shall not be construed as being limited to having the meaning defined in commonly used dictionaries. It will be further understood that the words or terms should be interpreted as having meanings that are consistent with their meanings in the context of the relevant art and the technical idea of the invention, based on the principle that an inventor may properly define the meaning of the words or terms to best explain the invention.

The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting of the present invention. The terms of a singular form may include plural forms unless the context clearly indicates otherwise.

It will be further understood that the terms “include”, “comprise”, or “have” when used in this specification, specify the presence of stated features, numbers, steps, elements, or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof.

In the present specification, D₅₀, D₁₀, and D₉₀ may be defined as a particle diameter respectively corresponding to 50%, 10%, and 90% of the volume accumulation in a particle diameter distribution curve of a particle. The D₅₀, D₁₀, and D₉₀ may be measured by, for example, a laser diffraction method. The laser diffraction method generally enables measurement of a particle diameter from a sub-micron region to several millimeters, so that results of high reproducibility and high resolution may be obtained.

Hereinafter, the present invention will be described in detail.

Negative Electrode Active Material

The present invention relates to a negative electrode active material, and specifically, to a negative electrode active material for a lithium secondary battery.

Specifically, the negative electrode active material of the present invention includes natural graphite, and has a ratio of D₉₀ to D₁₀, which is D₉₀/D₁₀, of 2.0 to 2.2, a tap density of 1.11 g/cm³ to 1.19 g/cm³, and a BET specific surface area of 2.02 m²/g to 2.30 m²/g.

The negative electrode active material of the present invention is a negative electrode active material containing natural graphite, which is characterized in that the D₉₀/D₁₀, tap density, and BET specific surface area thereof are controlled to be in the above range. The negative electrode active material is one from which micro-pores of natural graphite which cause an electrolyte solution side reaction and a cycle expansion problem are sufficiently removed and having a uniform particle size distribution. Thus, when applied to a negative electrode, packing between particles may be facilitated, the thickness of an electrode after coating may be maintained small to reduce thickness variation during the roll-pressing of the electrode to a target thickness, the electrode orientation value is small, and an electrode advantageous in electrode expansion may be manufactured, so that the cycle expansion problem caused by the charge/discharge of the negative electrode and the resulting increase in the thickness of the negative electrode may be effectively prevented. In addition, the negative electrode active material having the D₉₀/D₁₀, tap density, and BET specific surface area adjusted to be in the above range is one from which pores inside particles are sufficiently removed, so that the cycle expansion problem caused by an electrolyte solution side reaction may be prevented to an excellent level.

The negative electrode active material includes natural graphite.

Since the negative electrode active material uses natural graphite, there is an advantage of having high output and capacity when compared to cases using other carbon-based active materials such as artificial graphite. Specifically, the negative electrode active material may include natural graphite having a theoretical capacity of 360 mAh/g or greater.

The negative electrode active material may further include a carbon coating layer formed on the natural graphite.

The carbon coating layer may contribute to improving the structural stability of the active material. In addition, the carbon coating layer may reduce micro-pores present in natural graphite and reduce the BET specific surface area to a desired level, and thus, may effectively prevent a side reaction with an electrolyte solution.

In terms of preventing a side reaction with an electrolyte solution, sufficiently improving structural stability, and preventing lithium intercalation/de-intercalation inhibition due to excessive formation, the carbon coating layer may be included in a negative electrode active material in an amount of 2 wt % to 7 wt %, preferably 3 wt % to 6 wt %, more preferably 4 wt % to 5 wt %.

The negative electrode active material may be spherical. In the present specification, “spherical” is a concept that encompasses not only a complete spherical shape, but also a slightly deformed, but substantially spherical form.

When the negative electrode active material is spherical, packing between active material particles may be facilitated, so that the thickness expansion problem of a negative electrode active material caused by charge/discharge may be reduced to an even more excellent level.

The spherical negative electrode active material may be implemented from spherical natural graphite, or a negative electrode active material including the spherical natural graphite and a carbon coating layer formed on the spherical natural graphite.

The D₉₀/D₁₀ of the negative electrode active material, which is the ratio of D₉₀ to D₁₀, is 2.0 to 2.2.

By adjusting the D₉₀/D₁₀ to the above-described level, the negative electrode active material may have active material particles having a uniform particle size, and accordingly, when the negative electrode active material is applied to a negative electrode, the packing degree of the active material particles may be improved, and the deformation of the particles may be prevented when roll-pressing the negative electrode. Therefore, the mechanical expansion of the negative electrode and/or the cycle expansion caused by an electrolyte solution side reaction may be prevented.

If the D₉₀/D₁₀ of the negative electrode active material exceeds 2.2, packing between particles is not facilitated, and since pores between the particles increase, an electrolyte solution side reaction may be intensified and the thickness expansion of the negative electrode caused by charge/discharge may not be effectively prevented. Also, the volume expansion due to the charge/discharge of particles having a relatively large particle size may be partially responsible for intensifying the phenomenon of swelling. If the D₉₀/D₁₀ of the negative electrode active material is less than 2.0, there is a problem that it is practically difficult to implement a negative electrode active material having such a particle size distribution.

The D₉₀/D₁₀ of the negative electrode active material may preferably be 2.05 to 2.12, and when in the above range, the cycle expansion problem caused by an electrolyte solution side reaction may be prevented to an even more excellent level, and the intercalation/de-intercalation of lithium may be facilitated since the entry/exit path of lithium ions is shortened.

The tap density of the negative electrode active material is 1.11 g/cm³ to 1.19 g/cm³.

The tap density may be defined as an apparent density calculated by measuring the final volume obtained by filling the negative electrode active material into a container, and then vibrating the container for a specific number of times. Specifically, the tap density may be defined as an apparent density calculated by measuring the final volume obtained by filling 40 g of the negative electrode active material into a cylindrical container having a diameter of 30 mm and a capacity of 100 mL, and then vibrating the container up and down 1,000 times with an amplitude of 14 mm.

A negative electrode active material having a tap density in the above range may be evaluated as having improved spheric properties, and accordingly, when a negative electrode is manufactured using the negative electrode active material, the packing degree between particles of the active material is improved, and the mechanical stress received by active material particles when roll-pressing the electrode may be reduced, so that it is advantageous in terms of preventing the phenomenon of swelling.

If the tap density of a negative electrode active material is less than 1.11 g/cm³, the active material having such a tap density may be evaluated as being relatively less spheroidized, so when the negative electrode active material layer is coated, the thickness of an electrode becomes large, and the thickness variation during the roll-pressing to a target thickness after coating the negative electrode active material layer is large and pressure needs to be applied with excessive force. Therefore, an electrode manufactured by using the negative electrode active material having such a tap density has high electrode orientation and is disadvantageous in terms of expansion performance of a negative electrode. Meanwhile, when the tap density of a negative electrode active material is greater than 1.19 g/cm³, it is necessary to perform a plurality of spheronization processes on natural graphite in order to implement the above tap density, and in order to reduce the specific surface area of natural graphite which has been increased by the plurality of spheronization processes, a binder (for example, pitch, etc.) needs to be used in excess, so that output properties and charge/discharge efficiency may be excessively deteriorated.

Preferably, the tap density of the negative electrode active material may be 1.14 g/cm³ to 1.17 g/cm³. When in the above range, the cycle expansion problem caused by an electrolyte solution side reaction may be prevented to an even more excellent level, the degree of spheroidization of the negative electrode active material may be improved to a desirable level, and active material arrangement in a negative electrode may be well achieved, so that it is preferable in terms of shortening the movement path of lithium ions and facilitating the intercalation/de-intercalation of lithium.

The BET specific surface area of the negative electrode active material is 2.02 m²/g to 2.30 m²/g.

In general, natural graphite has a large specific surface area due to the presence of pores on the surface and inside thereof, and accordingly, has a problem in that a cycle expansion problem caused by an electrolyte solution side reaction intensifies. However, the negative electrode active material of the present invention adjusts a BET specific surface area range to the above-described level despite the use of natural graphite, and thus, may effectively prevent an electrolyte solution side reaction.

If the BET specific surface area of the negative electrode active material exceeds 2.30 m²/g, an electrolyte solution side reaction intensifies, and an irreversible reaction of the negative electrode active material increases, thereby decreasing efficiency, so that the output and capacity properties of natural graphite may not be sufficiently implemented. If the BET specific surface area of the negative electrode active material is less than 2.02 m²/g, the output properties of natural graphite may be deteriorated, and thus, is not preferable.

The BET specific surface area of the negative electrode active material may be 2.02 m²/g to 2.30 m²/g, preferably 2.05 to 2.15 m²/g. When in the above range, it is advantageous in terms of improving lifespan performance and high-temperature storage performance through preventing a swelling problem without deteriorating the output properties of natural graphite.

The BET specific surface area may be implemented by a process of adjusting the D₉₀/D₁₀ and tap density of the negative electrode active material to be in the above-described range and by a process of processing natural graphite through a cold isostatic press (CIP) method to be described later. Typically, natural graphite has a relatively large BET specific surface area due to pores present on the surface and inside thereof, and accordingly, has a problem in that a side reaction with an electrolyte solution intensifies. In addition, in order to reduce the high BET specific surface area of natural graphite, a method for forming excess carbon coating layer may be considered. However, the method causes the deterioration in the output properties of natural graphite, and thus, is not preferable. However, in the present invention, the D₉₀/D₁₀ and tap density of a negative electrode active material may be controlled to be in the above range and the amount and area of pores may be effectively controlled by a CIP process, and thus, the BET specific surface area of the negative electrode active material is lowered to a desired level to prevent a side reaction with an electrolyte solution, thereby improving the lifespan properties of the negative electrode active material and preferably implementing the high output properties of natural graphite.

The BET specific surface area of the negative electrode active material may be measured by, for example, a Brunauer-Emmett-Teller (BET) measurement method using BELSORP (BET equipment) of BEL JAPAN using an adsorption gas such as nitrogen.

The D₅₀ of the negative electrode active material is 6 μm to 15 μm. When the negative electrode active material has an average particle diameter (D₅₀) in the above range, the problem of specific surface area being increased and an electrolyte solution side reaction being intensified, which occurs when the average particle size is too small, is prevented, and the diffusion distance of lithium is maintained at a proper level, so that it is preferable in terms of exhibiting the output performance and fast charge performance of the natural graphite.

The D₅₀ of the negative electrode active material may be, preferably, 8 μm to 12 μm, more preferably 9.5 μm to 11.5 μm, and more preferably 10.2 μm to 10.8 μm. When in the above range, the packing performance of particles is improved and pores between particles are reduced, so that swelling prevention is excellent, and it is also preferred in terms of improving fast charge performance since lithium may be smoothly intercalated/de-intercalated even at a high C-rate.

Method for Preparing Negative Electrode Active Material

In addition, the present invention provides a method for preparing a negative electrode active material.

Specifically, the method for preparing a negative electrode active material of the present invention includes pressurizing natural graphite by a cold isostatic press method; and adjusting a ratio of D₉₀ to D₁₀, which is D₉₀/₁₀, of the pressurized natural graphite to be 2.0 to 2.2, wherein the natural graphite having the adjusted D₉₀/D₁₀ has a BET specific surface area of 2.02 m²/g to 2.30 m²/g and a tap density of 1.11 g/cm³ to 1.19 g/cm³.

The natural graphite may be spherical. The spherical natural graphite may be prepared by spheronizing flaky natural graphite, and when natural graphite is spherical, packing between active material particles may be facilitated, so that the thickness expansion problem of a negative electrode active material caused by charge/discharge may be reduced to an even more excellent level.

The flaky natural graphite is prepared from a natural graphite raw material (for example, collected from graphite ore), and specifically, may be prepared through processes such as pulverizing natural graphite raw material, removing impurities by base treatment and/or acid treatment, washing, drying, and sieving.

Spherical natural graphite may be prepared through the spheronization of the flaky natural graphite. Specifically, the spheronization may be performed using a vortex flow pulverizer.

The method for preparing a negative electrode active material of the present invention includes a step of pressurizing natural graphite by a cold isostatic press method. By the cold isostatic press method, micro-pores present in the negative electrode active material including natural graphite may be effectively removed or reduced, and the specific surface area of the natural graphite may be reduced to an appropriate level, so that a cycle expansion problem of a negative electrode active material prepared therefrom, the problem due to an electrolyte solution side reaction, may be effectively prevented.

The “cold isostatic press” method or CIP refers to a molding method in which a powder is filled in a mold and the powder is compressed in an infinite multi-axis direction by hydrostatic pressure. That is, according to a CIP process, a power may be compressed in an isostatic manner, so that micro-pores present in a molded negative electrode active material may be uniformly removed or reduced. The natural graphite pressurized by the CIP process has a decreased reaction area with an electrolyte due to the reduction in micro-pores. Thus, a side reaction with the electrolyte may be significantly reduced and the lifespan properties of a battery may be improved.

The pressurization may be performed at a pressure of 80 MPa to 150 MPa, preferably 85 MPa to 100 MPa. When in the above pressure range, it is preferable in that pores of particles may be filled to a required level, so that a side reaction with the electrolyte may be effectively prevented, and damage to the negative electrode active material which may be caused by using excessively high pressure may be prevented.

The pressurization may be performed for a time of 30 seconds to 1,200 seconds, preferably 50 seconds to 600 seconds, and more preferably 60 seconds to 300 seconds. When in the above range, it is preferable in that the BET specific surface area of the negative electrode active material may be reduced to a required level.

The method for preparing a negative electrode active material of the present invention may further include a step of forming a carbon coating layer on the natural graphite after pressurizing the natural graphite. The carbon coating layer may compensate for damage or cracks of natural graphite which may occur during pressurization, pulverization, sieving, acid/base treatment processes, and the like of the natural graphite, reduce the BET specific surface area of the natural graphite increased due to the pulverization and the like, and improve the mechanical stability of the negative electrode active material.

The carbon coating layer may be formed by providing at least one precursor selected from the group consisting of pitch, rayon and polyacrylonitrile-based resins to spherical natural graphite and performing heat-treatment to thermally decompose the precursor. A heat-treatment process for forming the carbon coating layer may be performed in the temperature range of 1,100° C. to 1,500° C., more preferably 1,200° C. to 1,300° C., in terms of forming a uniform carbon coating layer and preventing excessive formation of the carbon coating layer.

The carbon coating layer may be formed in an amount of 2 wt % to 7 wt %, preferably 3 wt % to 6 wt %, and more preferably 4 wt % to 5 wt % based on the total weight of the natural graphite and the carbon coating layer. In this case, lithium intercalation/de-intercalation inhibition due to the excessive formation of the carbon coating layer may be prevented while sufficiently improving an effect of preventing a side reaction with an electrolyte solution and structural stability.

The method for preparing a negative electrode active material of the present invention includes a step of adjusting the D₉₀/D₁₀ of the pressurized natural graphite, which is the ratio of D₉₀ to D₁₀, to be 2.0 to 2.2, preferably 2.05 to 2.12.

The negative electrode active material having the D₉₀/D₁₀ adjusted to be in the above-described ranges has a uniform particle size distribution. Thus, when applied to a negative electrode, the packing degree between particles is excellent, and the cycle expansion problem caused by the charge/discharge of the negative electrode and the resulting increase in the thickness of the negative electrode may be effectively prevented.

The step of adjusting the particle diameter distribution of the pressurized natural graphite may be performed by, for example, sieving, which is a well-known method in the art for adjusting a particle diameter distribution of particles.

When forming a carbon coating layer on the pressurized natural graphite, the step of adjusting a particle diameter distribution may be performed before forming the carbon coating layer or after forming the carbon coating layer.

The method for preparing a negative electrode active material of the present invention may further include a step of adjusting the D₅₀ of the pressurized natural graphite to 6 μm to 15 μm, preferably 8 μm to 12 μm, more preferably 9.5 μm to 11.5 μm, and even more preferably 10.2 μm to 10.8 μm. When the average particle diameter is adjusted to be in the above range, the packing performance of particles is improved and pores between particles are reduced, so that swelling prevention is excellent, and it is also preferred in terms of improving fast charge performance since lithium may be smoothly intercalated/de-intercalated even at a high C-rate due to the small particle size.

The step of adjusting the particle diameter distribution of the pressurized natural graphite may be performed by, for example, sieving, which is a well-known method in the art for adjusting a particle diameter distribution of particles.

When forming a carbon coating layer on the pressurized natural graphite, the step of adjusting a particle diameter distribution may be performed before forming the carbon coating layer or after forming the carbon coating layer.

The BET specific surface area of the natural graphite having the adjusted D₉₀/D₁₀, or natural graphite having a carbon coating layer is 2.02 m²/g to 2.30 m²/g. When in the above-described range, micro-pores present in the natural graphite are controlled to a required level, so that a side reaction with an electrolyte solution may be effectively prevented. The BET specific surface area of the natural graphite having the adjusted D₉₀/D₁₀, or natural graphite having a carbon coating layer may preferably be 2.05 to 2.15 m²/g.

The above BET specific surface area range may be implemented by, for example, pressurizing the natural graphite by the above-described cold isostatic press method, adjusting processing conditions of the cold isostatic press method, or adjusting the strength at the time of pulverization or crushing, the content of a precursor for forming a carbon coating layer, the heat-treatment temperature at the time of forming the carbon coating layer, and the like.

The tap density of the natural graphite having the adjusted D₉₀/D₁₀, or natural graphite having a carbon coating layer is 1.11 g/cm³ to 1.19 g/cm³. A negative electrode active material having a tap density in the above range may be evaluated as having an improved sphericity, and accordingly, when a negative electrode is manufactured using the negative electrode active material, the packing degree between particles of the active material is improved, and the mechanical stress received by active material particles when roll-pressing the electrode may be reduced, so that it is advantageous in terms of preventing the phenomenon of swelling.

Preferably, the tap density of the natural graphite having the adjusted D₉₀/D₁₀, or natural graphite having a carbon coating layer may be 1.14 g/cm³ to 1.17 g/cm³, and when the same is applied to a negative electrode, packing between particles may be facilitated, the thickness of an electrode after coating may be maintained small to reduce thickness variation during the roll-pressing of the electrode to a target thickness, the electrode orientation value is small, and an electrode advantageous in electrode expansion may be manufactured, so that the cycle expansion problem caused by the charge/discharge of the negative electrode, the phenomenon of swelling, and the resulting increase in the thickness of the negative electrode may be effectively prevented.

The above tap density range may be implemented by adjusting spheronization processing conditions of flaky natural graphite.

The D₉₀/D₁₀, BET specific surface area, and tap density of the negative electrode active material prepared in accordance with the method for preparing a negative electrode active material according to the present invention may be implemented by whether or not performing the above-described spheronization treatment process of natural graphite, pressurization by a cold isostatic press method, carbon coating layer formation process and/or sieving process, and adjusting conditions. The negative electrode active material prepared thereby is excellent in packing properties between particles and is prevented from the problem of cycle expansion caused by an electrolyte side reaction and the problem of thickness expansion of a negative electrode active material layer, and the output characteristics of natural graphite may be exhibited to an excellent level.

Negative Electrode

In addition, the present invention provides a negative electrode including the negative electrode active material described above.

Specifically, the negative electrode of the present invention includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, wherein the negative electrode active material layer includes the negative electrode active material described above.

The negative electrode current collector may typically have a thickness of 3 μm to 500 μm. The negative electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery, and for example, may include at least one selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, fired carbon, and an aluminum-cadmium alloy. In addition, the negative electrode current collector may have microscopic irregularities formed on the surface thereof, and the bonding force of the negative electrode active material may be enhanced thereby. In addition, the negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a non-woven fabric body.

The negative electrode active material layer includes the negative electrode active material described above.

The negative electrode active material layer may further include other active materials know in the art in the range that does not deteriorate the effect of the invention, specifically, one or two or more selected from the group consisting of a carbonaceous material, a metal such as a lithium-containing titanium composite oxide (LTO), Si, Sn, Li, Zn, Mg, Cd, Ce, Ni or Fe, an alloy composed of the metal, an oxide of the metal, and a composite of the metal and carbon.

The negative electrode active material may be included in the negative electrode active material layer in an amount of 80 wt % to 99 wt %, preferably 90 wt % to 98 wt %.

In addition, the negative electrode active material layer may further selectively include at least one additive selected from the group consisting of a binder, a thickener, and a conductive material, together with the negative electrode active material.

The binder is a component for assisting in bonding between a conductive material, an active material, and a current collector, and may be added in an amount of 1 wt % to 30 wt % in a negative electrode active material layer. Examples of the binder may include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, an ethylene-propylene-diene monomer (EPDM), a sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluorine rubber, various copolymers thereof, and the like.

As the thickener, any thickener used in a typical lithium secondary battery may be used, and one example is carboxymethyl cellulose (CMC).

The conductive material is a component for further improving the conductivity of a negative electrode active material, and may be added in an amount of 1 wt % to 20 wt % in a negative electrode active material layer. The conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fiber such as carbon fiber and metal fiber; fluorocarbon; metal powder such as aluminum powder, and nickel powder; a conductive whisker such as zinc oxide and potassium titanate; a conductive metal oxide such as titanium oxide; or a conductive material such as a polyphenylene derivative, and the like may be used. Specific examples of a commercially available conductive material may include acetylene black series of Chevron Chemical Company, Denka Black of Denka Singapore Private Limited, products of Gulf Oil Company, etc., Ketjen black, EC series of Armak Company, Vulcan XC-72 of Cabot Company, and Super P of Timcal Company.

Since the negative electrode according to the present invention includes the above-described negative electrode active material, packing among active material particles in the negative electrode may be smoothly achieved, and the problem of cycle expansion caused by an electrolyte side reaction and the problem of thickness expansion of a negative electrode active material layer may be effectively prevented. Accordingly, the output and capacity properties of natural graphite may be effectively exhibited, while improving the lifespan properties.

When performing X-ray diffraction analysis of the negative electrode, the orientation index for the area ratio I(004)/I(110) may be 9 to 15, preferably 9 to 12. When in the above range, the negative electrode active material may be expanded in multi-directions, so that it is preferable in terms of preventing the phenomenon of swelling and improving lifespan properties.

Secondary Battery

In addition, the present invention provides a lithium secondary battery including the above-described negative electrode for a secondary battery.

Specifically, the lithium secondary battery includes the negative electrode described above, a positive electrode facing the negative electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte.

The positive electrode may include a positive electrode current collector, and a positive electrode active material layer formed on the positive electrode current collector.

The positive electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, the positive electrode current collector may include at least one selected from the group consisting of stainless steel, aluminum, nickel, titanium, and fired carbon.

The positive electrode current collector may typically have a thickness of 3 μm to 500 μm.

The positive electrode active material layer is formed on the positive electrode current collector, and includes a positive electrode active material.

The positive electrode active material is a compound capable of reversible intercalation and de-intercalation of lithium, and specifically, may include a lithium composite metal oxide containing one or more metals such as cobalt, manganese, nickel or aluminum, and lithium. More specifically, the lithium composite metal oxide may be a lithium-manganese-based oxide (e.g., LiMnO₂, LiMn₂O₄, etc.), a lithium-cobalt-based oxide (e.g., LiCoO₂, etc.), a lithium-nickel-based oxide (e.g., LiNiO₂, etc.), a lithium-nickel-manganese-based oxide (e.g., LiNi_(1-Y)Mn_(Y)O₂ (wherein 0<Y<1), LiMn_(2-Z)Ni_(Z)O₄ (wherein 0<Z<2), etc.), a lithium-nickel-cobalt-based oxide (e.g., LiNi_(1-Y1)Co_(Y1)O₂ (wherein 0<Y1<1), etc.), a lithium-manganese-cobalt-based oxide (e.g., LiCo_(1-Y)Mn_(Y2)O₂ (wherein 0<Y2<1), LiMn_(2-Z1)Co_(z1)O₄(wherein 0<Z1<2, etc.), a lithium-nickel-manganese-cobalt-based oxide (e.g., Li(Ni_(p)Co_(q)Mn_(r1))O₂ (wherein 0<p<1, 0<q<1, 0 <r1<1, p+q+r1=1) or Li(Ni_(p1)Co_(q1)Mn_(r2))O₄ (wherein 0<p1<2, 0<q1 <2, 0<r2<2, p1+q1+r2=2, etc.), or a lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni_(p2)Co_(q2)Mn_(r3)A_(S2))O₂ (wherein M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are each an atomic fraction of stand-alone elements, wherein 0<p2<1, 0<q2<1, 0 <r3<1, 0<s2<1, p2+q2+r3+s2=1, etc.) and the like, and any one thereof or a mixture of two or more thereof may be included. Among these, due to the fact that the capacity properties and stability of a battery may be increased, the lithium composite metal oxide may be LiCoO₂, LiMnO₂, LiNiO₂, a lithium nickel-manganese-cobalt oxide (e.g., Li(Ni_(0.6)Mn_(0.2)Co_(0.2))O₂), Li(Ni_(0.5)Mn_(0.3)Co_(0.2))O₂, or Li(Ni_(0.8)Mn_(0.1)Co_(0.1))O₂, etc.), or a lithium nickel cobalt aluminum oxide (e.g., Li(Ni_(0.8)Co_(0.15)Al_(0.05))O₂, etc.), and the like.

The positive electrode active material may be included in an amount of 80 wt % to 99 wt % based on the total weight of a positive electrode active material layer.

The positive electrode active material layer may further selectively include at least one additive selected from the group consisting of a binder and a conductive material, together with the positive electrode active material described above.

The binder is a component for assisting in bonding of an active material, a conductive material, and the like, and in bonding to a current collector, and is typically added in an amount of 1 wt % to 30 wt % based on the total weight of a positive electrode active material layer. Examples of the binder may include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, an ethylene-propylene-diene monomer (EPDM), a sulfonated EPDM, styrene-butadiene rubber, fluorine rubber, various copolymers thereof, and the like.

The conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, graphite; a carbon-based material such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; a conductive fiber such as carbon fiber and metal fiber; fluorocarbon; metal powder such as aluminum powder, and nickel powder; a conductive whisker such as zinc oxide and potassium titanate; a conductive metal oxide such as titanium oxide; or a conductive material such as a polyphenylene derivative and the like may be used. Specific examples of a commercially available conductive material may include acetylene black series of Chevron Chemical Company, Denka Black of Denka Singapore Private Limited, products of Gulf Oil Company, etc., Ketjen black, EC series of Armak Company, Vulcan XC-72 of Cabot Company, and Super P of Timcal Company.

The conductive material may be included in an amount of 1 wt % to 30 wt % based on the total weight of a positive electrode active material layer.

The positive electrode active material layer may be prepared by adding an additive including a positive electrode active material and selectively, a binder and/or a conductive material to a solvent to prepare a positive electrode slurry, and then applying the slurry on the positive electrode current collector, followed by roll-pressing and drying.

The solvent may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount such that a preferred viscosity is achieved when the positive electrode active material and selectively, a binder and a conductive material, and the like are included. For example, the solvent may be included in an amount such that the concentration of a solid including the positive electrode active material and selectively a binder and a conductive material is 50 wt % to 95 wt %, preferably 70 wt % to 90 wt %.

In the lithium secondary battery, a separator is to separate the negative electrode and the positive electrode and to provide a movement path for lithium ions. Any separator may be used without particular limitation as long as it is a separator commonly used in a secondary battery. Particularly, a separator having excellent moisture-retention of an electrolyte as well as low resistance to ion movement in the electrolyte is preferable. Specifically, a porous polymer film, for example, a porous polymer film manufactured using a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, or a laminated structure having two or more layers thereof may be used. Also, a typical porous non-woven fabric, for example, a non-woven fabric formed of glass fiber having a high melting point, polyethylene terephthalate fiber, or the like may be used. Also, a coated separator including a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may be selectively used in a single-layered or a multi-layered structure.

In addition, the electrolyte used in the present invention may be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten-type inorganic electrolyte, and the like, all of which may be used in the manufacturing of a lithium secondary battery, but is not limited thereto.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

Any organic solvent may be used without particular limitation as long as it may serve as a medium through which ions involved in an electrochemical reaction of a battery may move. Specifically, as the organic solvent, an ester-based solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, and ϵ-caprolactone; an ether-based solvent such as dibutyl ether or tetrahydrofuran; a ketone-based solvent such as cyclohexanone; an aromatic hydrocarbon-based solvent such as benzene and fluorobenzene; a carbonate-based solvent such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); an alcohol-based solvent such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a linear, branched, or cyclic C2 to C20 hydrocarbon group and may include a double-bond, an aromatic ring or ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among the above solvents, a carbonate-based solvent is preferable, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having a high ionic conductivity and a high dielectric constant, which may increase charging/discharging performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethylmethyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferable. In this case, the performance of the electrolyte solution may be excellent when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9.

Any compound may be used as the lithium salt without particular limitation as long as it may provide lithium ions used in a lithium secondary battery. Specifically, as the lithium salt, LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiSbF₆, LiAlO₄, LiAlCl₄, LiCF₃SO₃, LiC₄F₉SO₃, LiN(C₂F₅SO₃)₂, LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)₂, LiCl, LiI, LiB(C₂O₄)₂, or the like may be used. The lithium salt may be used in a concentration range of 0.1 M to 2.0 M. When the concentration of the lithium salt is in the above range, the electrolyte has suitable conductivity and viscosity, thereby exhibiting excellent performance, and lithium ions may effectively move.

As described above, the secondary battery according to the present invention is useful for portable devices such as a mobile phone, a notebook computer, and a digital camera, and in the field of electric vehicles such as a hybrid electric vehicle (HEV), and in particular, may be preferably used as a component battery for a medium-to-large-sized battery module. Therefore, the present invention also provides a medium-to-large-sized battery module including the secondary battery as described above as a unit cell.

A medium-to-large-sized battery module as described above may be preferably applied to a power source which requires high output and large capacity, such as an electric vehicle, a hybrid electric vehicle, and a power storage device.

Hereinafter, Examples of the present invention will be described in detail so that those skilled in the art may easily carry out the present invention. However, the present invention may be embodied in many different forms, and is not limited to Examples set forth herein.

EXAMPLES AND COMPARATIVE EXAMPLES Example 1: Preparation of Negative Electrode Active Material

A natural graphite raw material was collected from graphite ore using flotation, and the natural graphite raw material was treated with acid or base to remove impurities, and then washed and dried to prepare flaky natural graphite. The flaky natural graphite obtained above was spheronized using a vortex flow pulverizer, treated with sulfuric acid to remove impurities therefrom, and then dried to prepare spherical natural graphite.

The spherical natural graphite was filled into a mold and pressurized through a cold isostatic press (CIP) method. During the pressurization, the pressurization pressure was 90 MPa, and pressurization was performed for 100 seconds.

The pressurized spherical natural graphite and a pitch having a softening point of 175 to 185° C. was mixed, and the mixture was heat-treated in an inert atmosphere at 1,250° C. for 24 hours in a dry manner to form a carbon coating layer on the spherical natural graphite.

The spherical natural graphite on which the carbon coating layer is formed was crushed, sieved, and de-ironed, and the 10% volume cumulative diameter D₁₀, 50% volume cumulative diameter D₅₀, and 90% volume cumulative diameter D₉₀ thereof in a particle size distribution were adjusted to be 7.4 μm, 10.7 μm, and 15.5 μm, respectively, to prepare a negative electrode active material of Example 1.

At this time, the carbon coating layer was formed to be 4.5 wt % based on the total weight of the negative electrode active material of Example 1.

Example 2: Preparation of Negative Electrode Active Material

A negative electrode active material was prepared in the same manner as in Example 1 except that in Example 1, the spheronization processing conditions and sieving conditions were differently applied, and the carbon coating layer was formed in 5 wt % based on the total weight of the negative electrode active material.

The D₁₀, D₅₀, D₉₀ of the negative electrode active material was 7.3 μm, 10.0 μm, and 14.6 μm, respectively.

Comparative Example 1: Preparation of Negative Electrode Active Material

A negative electrode active material was prepared in the same manner as in Example 1 except that in Example 1, the spheronization processing conditions and sieving conditions were differently applied, and the carbon coating layer was formed in 5 wt % based on the total weight of the negative electrode active material.

The D₁₀, D₅₀, D₉₀ of the negative electrode active material was 6.4 μm, 9.0 μm, and 13.6 μm, respectively.

Comparative Example 2: Preparation of Negative Electrode Active Material

A negative electrode active material was prepared in the same manner as in Example 1 except that in Example 1, the spheronization processing conditions and sieving conditions were differently applied, and the carbon coating layer was formed in 4 wt % based on the total weight of the negative electrode active material.

The D₁₀, D₅₀, D₉₀ of the negative electrode active material was 6.8 μm, 11.0 μm, and 19.1 μm, respectively.

Comparative Example 3: Preparation of Negative Electrode Active Material

A negative electrode active material was prepared in the same manner as in Example 1 except that in Example 1, the spheronization processing conditions and sieving conditions were differently applied, the carbon coating layer was formed in 5 wt % based on the total weight of the negative electrode active material, and pressurization by the cold isostatic press (CIP) method was performed for 400 seconds.

The D₁₀, D₅₀, D₉₀ of the negative electrode active material was 7.0 μm, 10.0 μm, and 15.4 μm, respectively.

Comparative Example 4: Preparation of Negative Electrode Active Material

A negative electrode active material was prepared in the same manner as in Example 1 except that in Example 1, the spheronization processing conditions and sieving conditions were differently applied, the carbon coating layer was formed in 5 wt % based on the total weight of the negative electrode active material, and pressurization by the cold isostatic press (CIP) method was not performed.

The D₁₀, D₅₀, D₉₀ of the negative electrode active material was 6.3 μm, 9.0 μm, and 15.5 μm, respectively.

Comparative Example 5: Preparation of Negative Electrode Active Material

A negative electrode active material was prepared in the same manner as in Example 1 except that in Example 1, the spheronization processing conditions and sieving conditions were differently applied, and the carbon coating layer was formed in 4 wt % based on the total weight of the negative electrode active material.

The D₁₀, D₅₀, D₉₀ of the negative electrode active material was 7.5 μm, 11.0 μm, and 16.5 μm, respectively.

Comparative Example 6: Preparation of Negative Electrode Active Material

A negative electrode active material was prepared in the same manner as in Example 1 except that in Example 1, the spherical natural graphite and an excessive amount of the pitch were mixed, heat-treatment after the mixing of the spherical natural graphite and the pitch was performed at 3,000° C. instead of 1,250° C., the carbon coating layer was formed in 10 wt % based on the total weight of the negative electrode active material, and pressurization by the cold isostatic press (CIP) method was not performed.

The D₁₀, D₅₀, D₉₀ of the negative electrode active material was 6.8 μm, 10.0 μm, and 14.8 μm, respectively.

The D₅₀, D₁₀, D₉₀, BET specific surface area, and tap density of the negative electrode active material prepared in each of Example 1, Example 2, and Comparative Examples 1 to 6 were respectively measured, and are shown in Table 1 below.

TABLE 1 Tap BET specific CIP density D₁₀ D₅₀ D₉₀ surface area performance (g/cm³) (μm) (μm) (μm) D₉₀/D₁₀ (m²/g) Example 1 ◯ 1.15 7.4 10.7 15.5 2.09 2.07 Example 2 ◯ 1.13 7.3 10.0 14.6 2.00 2.20 Comparative ◯ 1.06 6.4 9.0 13.6 2.13 2.10 Example 1 Comparative ◯ 1.01 6.8 11.0 19.1 2.81 2.20 Example 2 Comparative ◯ 1.04 7.0 10.0 15.4 2.20 2.40 Example 3 Comparative X 1.13 6.3 9.0 15.5 2.46 3.20 Example 4 Comparative ◯ 1.10 7.5 11.0 16.5 2.20 2.20 Example 5 Comparative X 1.20 6.8 10.0 14.8 2.18 2.00 Example 6

The tap density was obtained by calculating the apparent density by measuring the final volume obtained by filling 40 g of the negative electrode active material into a cylindrical container having a diameter of 30 mm and a capacity of 100 mL, and then vibrating the container up and down 1,000 times with an amplitude of 14 mm.

The BET specific surface area of a negative electrode active material was measured by pre-treating the negative electrode active material at 130° C., and then by a Brunauer-Emmett-Teller (BET) measurement method using BELSORP (BET equipment) of BEL JAPAN Co., Ltd, by using nitrogen gas.

EXPERIMENTAL EXAMPLES Manufacturing of Negative Electrode

The negative electrode active material prepared in Example 1, Super C65 as a conductive material, styrene butadiene rubber (SBR) as a binder, and carboxymethylcellulose (CMC) as a thickening agent were mixed in a weight ratio of 96.6:1:1.3:1.1, and then added with water to prepare a negative electrode slurry. Thereafter, the negative electrode slurry was applied on a copper foil, and then vacuum dried for 8 hours at about 130° C., followed by roll-pressing to manufacture a negative electrode having an area of 1.4875 cm². At this time, the negative electrode was manufactured to have a loading of 3.6 mAh/cm².

A negative electrode of each of Example 2, and Comparative Examples 1 to 6 was manufactured in the same manner as in Example 1 except that the negative electrode active material of each of Example 2, and Comparative Examples 1 to 6 was used instead of the negative electrode active material of Example 1.

Experimental Example 1: Swelling Evaluation Manufacturing of Secondary Battery

LiCoO₂ as a positive electrode active material, Li-435 (manufactured by Denka Co., Ltd) as a conductive material, KF9700 (manufactured by Kureha Co., Ltd) as a binder, and BH-730H (manufactured by Zeon Co., Ltd) as a thickener were mixed in a weight ratio of 97.68:1.20:1.00:0.12, and then added with N-methylpyrrolidone (NMP) as a solvent to prepare a positive electrode slurry, and the positive electrode slurry was applied on an aluminum foil, and then vacuum dried for 8 hours at about 130° C., followed by roll-pressing to manufacture a positive electrode having an area of 1.7671 cm². At this time, the positive electrode was manufactured to have a loading of 3.4 mAh/cm².

A polyolefin separator was interposed between the negative electrode manufactured in each of Example 1, Example 2, and Comparative Examples 1 to 6 and the positive electrode, and then an electrolyte solution was injected thereto to manufacture a full-cell secondary battery of each of Examples and Comparative Examples. The electrolyte solution used was prepared by adding vinylene carbonate (VC) to a non-aqueous electrolyte solvent in an amount of 0.5 wt % based on the solvent, the solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 1:4, and then dissolving 1 M of LiPF₆ therein.

Swelling Evaluation

The lithium secondary battery manufactured in each of Example 1, Example 2, and Comparative Examples 1 to 6 was charged and discharged at 0.1 C for the first cycle, 0.2 C for the second cycle, and 0.5 C from the third cycle to the 30-th cycle by having a charging range of SOC 0 to SOC 95. Thereafter, a swelling ratio was calculated by Equation 1 below.

Swelling ratio (%)={(t ₂ −t ₁)/t ₁}×100   [Equation 1]

(t1 is the thickness of a negative electrode for a secondary battery before the first charge/discharge cycle, t2 is the thickness of a negative electrode for a secondary battery after the 30^(th) charge/discharge cycle)

TABLE 2 Swelling ratio (%) Example 1 21 Example 2 22 Comparative Example 1 24 Comparative Example 2 26 Comparative Example 3 29 Comparative Example 4 26 Comparative Example 5 27 Comparative Example 6 30

Referring to Table 2, the negative electrode active material of each of Example 1 and Example 2 in which natural graphite was pressurized by the cold isostatic press method and the D₉₀/D₁₀, tap density, and BET specific surface area of the negative electrode active material were adjusted to a desired range, was prevented from being subjected to an electrolyte solution side reaction, and was also prevented from being subjected to the phenomenon of swelling to an excellent level when compared to Comparative Examples.

Experimental Example 2: Fast Charge Performance Evaluation Manufacturing of a Coin-Type Half-Cell

The negative electrode manufactured in each of Example 1, Example 2, and Comparative Examples 1 to 6 was punched to a size of a coin cell and then a polyolefin separator was interposed between the negative electrode and a lithium foil which is a counter electrode. Thereafter, an electrolyte solution in which LiPF₆ of 1 M was dissolved in a solvent in which ethylene carbonate (EC) and ethylmethyl carbonate (EMC) were mixed in a volume ratio of 20:80 and 0.5 wt % of a vinylene carbonate (VC) additive was added thereto was injected to manufacture a coin-type half-cell of each of Examples and Comparative Examples.

Fast Charge Performance Evaluation

In order to confirm the fast charge properties of the negative electrodes prepared above and according to Examples 1 and 2 and Comparative Examples 1 to 6. a Li-plating experiment was performed.

The coin-type half-cell of each of Examples and Comparative Examples was charged and discharged at 0.1 C for 3 cycles, and then charged at 3 C for 15 minutes to identify the inflection point of a Profile at the first derivation and dQ/dV to quantify Li-plating SOC (%), which is the SOC at the time when lithium precipitation occurs on the surface of the negative electrode. The results are shown in Table 3.

TABLE 3 Li-Plating SOC (%) Example 1 41 Example 2 42 Comparative Example 1 41 Comparative Example 2 37 Comparative Example 3 41 Comparative Example 4 41 Comparative Example 5 38 Comparative Example 6 36

Referring to Table 3, the negative electrodes of Example 1 and Example 2 exhibit excellent fast charge performance compared to those of Comparative Examples 2, 5, and 6.

Although Comparative Examples 1, 3, and 4 show the same level of fast charge performance as Example 1, the swelling prevention performance thereof is poor compared to Examples as described above, which is not preferable. 

1. A negative electrode active material comprising: natural graphite, wherein the negative electrode active material has a D₉₀/D₁₀, which is the ratio of D₉₀ to D₁₀, of 2.0 to 2.2; a tap density of 1.11 g/cm³ to 1.19 g/cm³; and a BET specific surface area of 2.02 m²/g to 2.30 m²/g.
 2. The negative electrode active material of claim 1, wherein a D₅₀ of the negative electrode active material is 6 μm to 15 μm.
 3. The negative electrode active material of claim 1, wherein the D₉₀/D₁₀ of the negative electrode active material is 2.05 to 2.12.
 4. The negative electrode active material of claim 1, further comprising a carbon coating layer on the natural graphite.
 5. The negative electrode active material of claim 4, wherein the carbon coating layer is present in the negative electrode active material in an amount of 2 wt % to 7 wt %.
 6. The negative electrode active material of claim 1, wherein the negative electrode active material is spherical.
 7. A method for preparing a negative electrode active material, the method comprising: pressurizing natural graphite by a cold isostatic press method to obtain pressurized natural graphite; and adjusting a D₉₀/D₁₀ of the pressurized natural graphite, which is a ratio of D₉₀ to D₁₀, to be 2.0 to 2.2, wherein the natural graphite having the adjusted D₉₀/D₁₀ has a BET specific surface area of 2.02 m²/g to 2.30 m²/g and a tap density of 1.11 g/cm³ to 1.19 g/cm³.
 8. The method of claim 7, further comprising forming a carbon coating layer on the natural graphite after pressurizing the natural graphite.
 9. The method of claim 8, wherein the carbon coating layer is present in an amount of 2 wt % to 7 wt % based on a total weight of the natural graphite and the carbon coating layer.
 10. The method of claim 7, wherein the pressurization is performed at a pressure of 80 MPa to 150 MPa.
 11. The method of claim 7, wherein the pressurization is performed for a time of 30 seconds to 1,200 seconds.
 12. A negative electrode comprising: a negative electrode current collector; and a negative electrode active material layer on the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode active material according to claim
 1. 13. A secondary battery comprising: the negative electrode according to claim 12; a positive electrode facing the negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte. 